This is the second competing renewal of CA68837, an investigation aimed at the long-term goal of understanding the INK4 family of CDK inhibitors. Looking back at the journey over the past 10 years, we are pleased to see the contributions we made and, more remarkably, the advancement achieved collectively by others working in this field. The focus of our own research, following the initial discovery of p16Ink4a in 1993, has moved forward from the discovery and biochemical characterization of three additional INK4 genes during the first funding period (1994 - 1999), to the creation and histological analyses of various Ink4 mutant mouse strains during the second funding period (2000 - present). Built upon the large volume of phenotypical and pathological information we accumulated over the past several years, the research outlined in this application will move toward achieving the following two long-term goals: (1) developing and characterizing mouse models for human lung and breast cancers, and (2) to determine the mechanism and regulation of INK4 genes in stem cell control. Combining our strength and success in genetic and tumor analysis in mice, biochemical characterization of CDK activities, and cellular studies of G1 progression, we propose three aims to achieve these two goals. (1) To determine the function and mechanism of p18lnk4c and Men1 in lung tumor suppression and stem cell control. (2) To determine how p18lnk4c and Brca1 functionally collaborate to suppress mammary tumors and regulate mammary stem cell expansion. (3) To determine the mechanisms regulating p16 gene expression. Three features of this investigation are: (1) Extensive use of genetically engineered mice and their derived isogenic cells as the primary experimental system, providing a highly sophisticated physiological setting to examine the function of INK4 family genes. (2) A combination of investigation of two INK4 genes with demonstrated tumor suppression function in one study would not only provide a comprehensive view of their individual function, but also uncover possible functional compensation and allow a comparison of their regulatory mechanisms. (3) The critical importance of this gene family in tumor suppression, stem cell control, and development of mouse models for different type of human cancers.